Heat shrinkable polymer films have gained substantial acceptance for such uses as the packaging of meats. This description will discuss the usage of films for packaging meat; it being understood that these films are also suitable for packaging other products. Some of the films embodying this invention are normally used as heat shrinkable bags supplied to a meat packer with one open end, to be closed and sealed after insertion of the meat. After the product is inserted, air is normally evacuated, the open end of the bag is closed, such as by heat sealing or applying a metal clip, and finally heat is applied, such as by hot water or hot air, to initiate shrinkage about the meat product.
In subsequent processing of the meat, the bag may be opened and the meat removed for further cutting of the meat into user portions, for retail sale, for example, or for institutional use.
Successful shrink bags must satisfy a multiplicity of requirements imposed by both the bag producer and the bag user. Of primary importance to the bag user is the capability of the bag to survive physically intact the process of being filled, evacuated, sealed closed, and heat shrunk. The bag must also be strong enough to survive the material handling involved in moving the contained product along the distribution system to the next processor, or to the user. Thus, the bag must physically protect the product.
It is also highly desirable to the bag user that the bag serve as a barrier to infusion of gaseous materials from the surrounding environment. Of particular importance is provision of an effective barrier to infusion of oxygen, since oxygen is well known to cause spoilage of food type products.
The bag producer requires a product which can be produced competitively, while meeting the performance requirements of the user. Thus the bag material should be relatively inexpensive to purchase, should be readily extrudable, and susceptible to orientation, with sufficient leeway in process parameters as to allow for efficient film production. The process should also be susceptible to extended production operations.
The orientation temperature should be a temperature which is economically achieved by the producer, and which provides for use of economical shrink processes by the bag user.
During fabrication and use of the film, it must be tough enough to withstand the various high temperature operations to which it is subjected, including heat sealing, and in some cases shrinking. Thus, its strength at high temperature, hereinafter referred to as hot strength, is an important consideration.
Conventional shrink bags have generally been constructed with ethylene vinyl acetate copolymers (EVA). In some cases the bags contain a layer of a vinylidene chloride-vinyl chloride copolymer (VDC-VC) to serve as an oxygen barrier. Ethylene vinyl alcohol copolymer (EVOH) is also known as an oxygen barrier material.
Notwithstanding the several shrink films which are available, and the advantages of shrink packaging, shrink packaging is not without its difficulties, many of which are attributable to limitations in the film. As will be appreciated, the processes of stretching the film, and later shrinking it, expose the film to rather severe conditions, due to the nature of the operations.
It is especially important to appreciate that the film is particularly vulnerable to failure at conditions of operation, due to the high temperatures to which it is exposed in the orientation and shrinking processes. The film must be susceptible to orientation without distortion or separation of the multiple layers which are normally present in films of this nature. The film must be strong enough, at the orientation temperature to withstand the stresses of stretching without the creation of holes, tears, or non-uniform zones of stretching.
In the case of tubularly oriented films, the film must be capable of supporting the stretching bubble during the orientation process. Finally, each of the layers of the film should be susceptible to orientation without fracture, separation, or creation of holes in the layer.
In packaging use, the film must respond to heat rapidly enough in the shrinking process for commercial practicality and yet must not exhibit such a level of shrink energy as would cause the film to pull apart or delaminate during shrinkage, under its own internal forces. Moreover, the shrink related problems are increased when the contained product, such as a cut of meat includes protrusions such as bones, and/or significant cavities in its surface.
Particularly in the case of cavities in the product, such as around the interior of the rib section in a cut of meat, the redistribution of an area of the film adjacent the cavity places an extra strain on the ability of the film to conform to the product in the shrinking process while maintaining film continuity.
Another area where film packages are known to be susceptible to failure is at any area where portions of the film are sealed to each other by a heat seal. In the formation of a heat seal, at least portions of the film are heated to a temperature where they are soft enough to flow and be melt merged when simultaneously subjected to pressure. It is desirable to be able to form heat seals in a film over a range of temperatures and pressures so that commerical processes can fluctuate within the normal operating parameters. Whatever the acceptable range of conditions of formation of heat seals, it is critical that the seals have adequate strength to hold the package closed, and prevent leakage into or out of the package until it is intentionally opened. Thus the strength of heat seals is also one of the important measures of the value of films which are used in applications where heat seals are formed.
The common factor in all these situations is that the film is heated to a high temperature, at which it may be softened, and an operation is performed, usually by deformation such as stretching, shrinking, and softening and merging to form a heat seal. While the film needs to be sufficiently deformable to perform a desired function, it need also have sufficient hot strength to not become so soft that it flows uncontrollably and assumes undesired shapes, such as by melting, developing holes, and the like.
It is generally known that cross-linking of polymer films improves their toughness and hot strength. It is known as a process to cross-link one layer of a multiple layer film containing a VDC-VC copolymer. This cross-linking of a single layer of a multiple layer film consists of a plurality of steps. For example, first the layer to be cross-linked is formed. Second, the formed layer is cross-linked. Third, additional layers are added to the cross-linked layer, as by extrusion coating, to form a multiple layer film. Finally, the multiple layer film is heated to orientation temperature and oriented. While this process may produce a functional film, it would be desirable to invent a process which might be less complex, require fewer steps, perhaps improve the inter-layer adhesion, and perhaps be more economical.
As regards the above described known process, it is seen that only one of the layers is cross-linked. A typical film has two outer layers of EVA and an inner layer, between the two EVA layers, of VDC-VC copolymer. One of the EVA layers is cross-linked and the other is not. With a plurality of processing steps required to form a film by the above-iterated known process, it is seen that processing economics might be attained by a different process, particularly if the number of processing steps can be reduced.
It is an object of this invention to provide improved film structures for use in packaging, especially for use in polymeric bags; and process for making film structures and packages. It is a special object to make films having improved properties for packaging uses, and to make them by processes which are competitive and economical as compared to previously available processes.